The invention is related to a driving device of a cold cathode fluorescent lamp (CCFL), and more particularly, to a driving device of a CCFL that quickly provides stable working illumination.
CCFLs are widely used in electronic devices such as scanners, LCD panels, notebook PCs and LCD televisions. Conventionally a CCFL circuit receives a static voltage equaling a working voltage of the CCFL. The CCFL may take as long as about three minutes after power supply is initialized to reach required functional illumination due to mercury vaporization in the CCPL not being sufficient at low temperatures, requiring the CCFL to warm up.
A driving method is disclosed by Johnson, et al. (U.S. Pat. No. 5,907,742, “Lamp control scheme for rapid warm-up of fluorescent lamp in office equipment”), in which a lamp is overdriven by high current within a predetermined time limit to accelerate mercury vaporization. The drive current is then reduced to a normal level. Scanner light output is monitored by a sensor circuit, and the system waits until the light output reaches a minimum and the light profile is sufficiently stable. The scanner is then calibrated to a white reference, and the closed loop control of the signal level is activated.
FIG. 1A shows a voltage vs. time relationship of a CCFL circuit according to Johnson's patent. FIGS. 1B-1D show illumination vs. time relationships of the CCFL associated with FIG. 1A, which shows a first voltage exceeding a working voltage of CCFL delivered to a CCFL circuit. The delivery period of the first voltage is from time 0 to time B. FIGS. 1B-1D show initialization and illumination rise of CCFL when the first voltage is provided. The illumination of the CCFL exceeds a working illumination at time A. Since the CCFL circuit receives the first voltage exceeding the working voltage of the CCFL, the temperature of the CCFL does not yet exceed a working temperature of the CCFL. Since the first voltage is still provided, the temperature of the CCFL, and in turn the illumination of the CCFL, continue to rise.
At time B, the working voltage of CCFL is provided. If the temperature of CCFL equals the working temperature of the CCFL at this time, the illumination transient of the CCFL is as shown in FIG. 1B. If the temperature of CCFL exceeds the working temperature of the CCFL at this time, the illumination transient of the CCFL is as shown in FIG. 1C; the illumination of CCFL decreases gradually to the working illumination with the temperature of the CCFL. If the temperature of CCFL is lower than the working temperature of the CCFL at this time, the illumination transient of the CCFL is as shown in FIG. 1D; the illumination of CCFL increases gradually to the working illumination with the temperature of CCFL.
Although Johnson provides faster warm-up of the CCFL, when the illumination exceeds the working illumination, as illustrated at time A in FIGS. 1B-1D, the CCFL is still not ready, since the illumination increases until at least time C. Additionally, if the period supplying the first voltage is fixed, illumination of the CCFL may still be higher or lower than the working illumination and not stable when the working voltage is provided, as illustrated at time C of FIG. 1C-1D.